This invention relates generally to holography and more particularly has reference to slant fringe holograms having increased efficiency and a method for making the same.
A hologram is a diffraction optical element which generally consists of a thin layer of photoreactive gelatin (e.g., dichromated gelatin) typically mounted on a glass substrate. The gelatin is exposed to intersecting laser beams which set up an interference pattern That pattern is recorded in the gelatin as a sinusoidal modulation in the index of refraction A pattern of holographic fringes corresponding to the refractive index modulation defines the hologram After the fringes are recorded, the gelatin is subjected to a chemical swelling procedure which amplifies the refractive index modulation to increase the diffraction efficiency of the hologram
A hologram acts as a wavelength and direction sensitive optical filter. Incident light within a predetermined range of wavelengths and directions will be diffracted by the hologram into a predetermined pattern and direction. For light significantly outside those wavelengths or directions, the hologram will act as a transparent piece of glass.
Holograms have many uses.
In modern aircraft, both military and commercial, a hologram is used in the combiner element of the head-up display. A head-up display provides relevant symbology, such as flight data and weapons aiming information, superimposed on the pilot's forward field of view. The symbology is generated on a cathode ray tube and projected through a relay lens to a transparent combiner element located between the pilot's eyes and the aircraft windscreen. The combiner includes a holographic film which diffracts the projected symbology to the pilot's eyes while simultaneously affording him an unobstructed view through the combiner to the outside world. See, U.S. Pat. No. 3,940,204 to Roger J. Withrington.
Holograms are also used in visors designed to protect eyes from damaging light exposure, such as might be encountered from an incoming laser beam. A holographic element in the visor diffracts and redirects incoming laser light out of the wearer's field of vision. By suitable control of the light used to generate the hologram (angles, wavelengths, etc.), the degree of reflectivity/transmissivity of the hologram can be selectively determined and a cone of total reflection for a given wavelength can be provided to protect the wearer's eyes from damaging incident light. It is essential that the hologram be nearly 100% efficient in diffracting the damaging light so that it does not reach the eye. With an extremely intense beam, such as a laser beam, inefficiently diffracted light may enter the cone of protection and be sufficient to damage the wearer's eyes.
It is usually desirable to construct a hologram with zero degree fringes, i.e., fringes that are parallel to the surface of the gelatin. However, in some cases, physical design restraints prevent the substrate holding the gelatin layer from being shaped in conformity with the desired fringe pattern. In those cases, the fringes intersect the surface of the gelatin to form a slant fringe pattern. One problem with slant fringe patterns is that their diffraction efficiency is inherently lower than corresponding zero degree fringe patterns. The problem compounds in holograms used in head-up displays because the fringe angle, and consequently the diffraction efficiency, often varies across the combiner.
There is no previously known technique for making slant fringe holograms having the very high efficiencies obtainable in zero degree fringe hologram. Any of the known procedures for making higher efficiency zero degree fringe holograms will produce increased efficiencies when applied to slant fringe holograms. Such procedures include optimizing exposure levels, processing temperatures, processing solutions, gelatin moisture content, dichromate concentration, beam ratios, gelatin thickness and gelatin types. However, slant fringe holograms will always have a lower efficiency than corresponding zero degree fringe holograms.
It is apparent that any application where diffraction efficiency (either in absolute value or uniformity across the format) is of importance, such as head-up displays, and in which optical design dictates slant fringes, would benefit from a technique which gives slant fringe holograms an efficiency approaching that of zero degree fringe holograms.